Memory devices are often used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized into various types, each type having associated with it various advantages and disadvantages.
Due, at least in part, to a rapidly growing numbers of compact, low-power portable computer systems and hand-held appliances in which stored information changes regularly, low energy read/write semiconductor memories have become increasingly desirable and widespread. Furthermore, because these portable systems often require data storage when the power is turned off, non-volatile storage devices are desired for use in such systems.
Recently, programmable metallization cell (PMC) devices have been developed for use in such systems. PMC devices offer advantages over traditional memory devices because PMC devices can be formed using amorphous material and can thus be added to existing devices formed on a semiconductor substrate. The PMC devices also typically have lower production cost and can be formed using flexible fabrication techniques, which are easily adaptable to a variety of applications. Further, the PMC devices may be scaled to less than a few square microns in size, the active portion of the device being less than on micron. This provides a significant advantage over traditional semiconductor technologies in which each device and its associated interconnect can take up several tens of square microns.
FIG. 1 illustrates a typical PMC device 100 formed on a surface of a substrate 110. Device 100 includes electrodes 120 and 130, an ion conductor 140, and an electrode 180. Generally, device 100 is configured such that when a bias greater than a threshold voltage (VT) is applied across electrodes 120 and 130, the electrical properties of structure 100 change. For example, as a voltage V≧VT is applied across electrodes 120 and 130, conductive ions within ion conductor 140 begin to migrate and form a conductive region (e.g., electrodeposit 160) at or near the more negative of electrodes 120 and 130. As the electrodeposit forms, the resistance between electrodes 120 and 130 decreases, and other electrical properties may also change. If the same voltage is applied in reverse, the electrodeposit will dissolve back into the ion conductor and the device will return to its high resistance state.
Ion conductor 140 may include small nanostructures that are rich with metal, which are super ionic phases. The distance between these structures is typically very small, allowing the dendritic growth to occur rapidly. Therefore, it can be inferred that the speed of programming is generally dependent on the distance between the nanostructures.
Because PMC devices have advantages over traditional semiconductor memory devices and can be used in a wide variety of applications, improved circuits for reading, writing, and erasing PMC devices are desired.